Evidence for a proto-black hole and a double astrophysical component in GRB 101023

Bianco

et al. 2013

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Context. Gamma-ray burst (GRB) 110709B is the first source for which Swift-BAT was triggered twice, with a time separation of ∼10 min. The first emission (called here episode 1) lasted from 40 s before the first trigger time until 60 s after it. The second emission (hereafter episode 2) lasted from 35 s before the second trigger time until 100 s after it. These features reproduce those of GRB 090618, which has recently been interpreted within the induced gravitational collapse (IGC) paradigm. In line with this paradigm, we assume the progenitor to be a close binary system composed of the core of an evolved star and a neutron star (NS). The evolved star explodes as a supernova (SN) and ejects material that is partially accreted by the NS. We identify this process with episode 1. The accretion process accumulates more than the critical mass of the NS, which gravitationally collapses into a black hole (BH). This process leads to the GRB emission, episode 2. The two trigger episodes have for the first time provided the possibility to cover the X-ray emission observed by XRT both prior to and during the prompt phase of GRB 110709B. Aims. We analyze the spectra and time variability of episodes 1 and 2 and compute the relevant parameters of the binary progenitor, as well as the astrophysical parameters both in the SN and the GRB phase in the IGC paradigm. Methods. We performed a time-resolved spectral analysis of episode 1 by fitting the spectrum with a blackbody (BB) plus a power-law (PL) spectral model. From the BB fluxes and temperatures of episode 1 and the luminosity distance d L , we evaluated the evolution with time of the radius of the BB emitter, associated here to the evolution of the SN ejecta. We analyzed episode 2 within the fireshell model, identifying the proper GRB (P-GRB) and simulating the light curve and spectrum. We established the redshift to be z = 0.75, following the phenomenological methods described in the literature, and our analysis of the late X-ray afterglow. It is most remarkable that the determination of the cosmological redshift on the basis of scaling the late X-ray afterglow, which was already verified in GRB 090618 and GRB 101023, is again verified by this analysis. Results. We find for episode 1 a temperature of the BB component that evolves with time following a broken PL, with the slope of the PL at early times α = 0 (constant function) and the slope of the PL at late times β = −4 ± 2. The break occurs at t = 41.21 s. The total energy of episode 1 is E (1) iso = 1.42 × 10 53 erg. The total energy of episode 2 is E (2) iso = 2.43 × 10 52 erg. We find at transparency a Lorentz factor Γ ∼ 1.73 × 10 2 , laboratory radius of 6.04 × 10 13 cm, P-GRB observed temperature kT P−GRB = 12.36 keV, baryon load B = 5.7 × 10 −3 and P-GRB energy of E P−GRB = 3.44 × 10 50 erg. We find a remarkable coincidence of the cosmological redshift by scaling the XRT data and with three other phenomenological methods. Conclusions. We interpret GRB 110709B as a member of the IGC sources, together with G...

Section: Episodesupporting

confidence: 89%

Section: Amati Relationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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GRB 110709B in the induced gravitational collapse paradigm

Penacchioni

Bianco

et al. 2013

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“…Such a process, if confirmed, will naturally explain the observations of the X-ray outburst 080109/SN 2008D (Soderberg et al 2008). This concept is very similar to the one of a proto-black hole as introduced in Ruffini et al ( , 2010a; Izzo et al (2011);Penacchioni et al (2012), where the emission from the collapsing core is clearly distinguished from the GRB. In that case, the collapsing core leads to the formation of the black hole, while in the present case it forms a neutron star.…”

Section: Late X-ray Emission In Grbs Associatedmentioning

confidence: 70%

Cooling of young neutron stars in GRB associated to supernovae

Negreiros

Bianco

et al. 2012

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Context. The traditional study of neutron star cooling has been generally applied to quite old objects such as the Crab Pulsar (957 years) or the central compact object in Cassiopeia A (330 years) with an observed surface temperature ∼10 6 K. However, recent observations of the late (t = 10 8 -10 9 s) emission of the supernovae (SNe) associated to GRBs (GRB-SN) show a distinctive emission in the X-ray regime consistent with temperatures ∼10 7 -10 8 K. Similar features have been also observed in two Type Ic SNe SN 2002ap and SN 1994I that are not associated to GRBs. Aims. We advance the possibility that the late X-ray emission observed in GRB-SN and in isolated SN is associated to a hot neutron star just formed in the SN event, here defined as a neo-neutron star. Methods. We discuss the thermal evolution of neo-neutron stars in the age regime that spans from ∼1 min (just after the proto-neutron star phase) all the way up to ages <10-100 yr. We examine critically the key factor governing the neo-neutron star cooling with special emphasis on the neutrino emission. We introduce a phenomenological heating source, as well as new boundary conditions, in order to mimic the high temperature of the atmosphere for young neutron stars. In this way we match the neo-neutron star luminosity to the observed late X-ray emission of the GRB-SN events: URCA-1 in GRB980425-SN1998bw, URCA-2 in GRB030329-SN2003dh, and URCA-3 in GRB031203-SN2003lw. Results. We identify the major role played by the neutrino emissivity in the thermal evolution of neo-neutron stars. By calibrating our additional heating source at early times to ∼10 12 -10 15 erg/g/s, we find a striking agreement of the luminosity obtained from the cooling of a neo-neutron stars with the prolonged (t = 10 8 -10 9 s) X-ray emission observed in GRB associated with SN. It is therefore appropriate a revision of the boundary conditions usually used in the thermal cooling theory of neutron stars, to match the proper conditions of the atmosphere at young ages. The traditional thermal processes taking place in the crust might be enhanced by the extreme high-temperature conditions of a neo-neutron star. Additional heating processes that are still not studied within this context, such as e + e − pair creation by overcritical fields, nuclear fusion, and fission energy release, might also take place under such conditions and deserve further analysis. Conclusions. Observation of GRB-SN has shown the possibility of witnessing the thermal evolution of neo-neutron stars. A new campaign of dedicated observations is recommended both of GRB-SN and of isolated Type Ic SN.

A double component in GRB 090618: a proto-black hole and a genuinely long gamma-ray burst

Izzo

Penacchioni

et al. 2012

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114

Context. The joint X-ray and gamma-ray observations of GRB 090618 by very many satellites offer an unprecedented possibility of testing crucial aspects of theoretical models. In particular, they allow us to test (a) in the process of gravitational collapse, the formation of an optically thick e + e − -baryon plasma self-accelerating to Lorentz factors in the range 200 < Γ < 3000; (b) its transparency condition with the emission of a component of 10 53−54 baryons in the TeV region and (c) the collision of these baryons with the circumburst medium (CBM) clouds, characterized by dimensions of 10 15−16 cm. In addition, these observations offer the possibility of testing a new understanding of the thermal and power-law components in the early phase of this GRB. Aims. We test the fireshell model of GRBs in one of the closest (z = 0.54) and most energetic (E iso = 2.90 × 10 53 erg) GRBs, namely GRB 090618. It was observed at ideal conditions by several satellites, namely Fermi, Swift, Konus-WIND, AGILE, RT-2, and Suzaku, as well as from on-ground optical observatories. Methods. We analyzed the emission from GRB 090618 using several spectral models, with special attention to the thermal and powerlaw components. We determined the fundamental parameters of a canonical GRB within the context of the fireshell model, including the identification of the total energy of the e + e − plasma, E e + e − tot , the proper GRB (P-GRB), the baryon load, the density and structure of the CBM. Results. We find evidence of the existence of two different episodes in GRB 090618. The first episode lasts 50 s and is characterized by a spectrum consisting of a thermal component, which evolves between kT = 54 keV and kT = 12 keV, and a power law with an average index γ = 1.75 ± 0.04. The second episode, which lasts for ∼100 s, behaves as a canonical long GRB with a Lorentz gamma factor at transparency of Γ = 495, a temperature at transparency of 29.22 keV and with a characteristic size of the surrounding clouds of R cl ∼ 10 15−16 cm and masses of ∼10 22−24 g. Conclusions. We support the recently proposed two-component nature of GRB 090618, namely, episode 1 and episode 2, with a specific theoretical analysis. We furthermore illustrate that episode 1 cannot be considered to be either a GRB or a part of a GRB event, but it appears to be related to the progenitor of the collapsing bare core, leading to the formation of the black hole, which we call a "proto-black hole". Thus, for the first time, we are witnessing the process of formation of a black hole from the phases just preceding the gravitational collapse all the way up to the GRB emission.